Mainly, I have "sensual" kids - every one of them has a hard time sleeping without a warm body right next to them, and they are all highly touch-happy (nipple twiddling & skin-rubbing is a big problem with me, because my kids can never control their hands when they nurse, and actually have screaming fits if I try to restrain their hand.) Because of that, if they get moved into a spot that is not mommy-temperature, they wake right up. This has been consistent with all 4 of my kids - I can get them to sleep in bed and then gently move away from them if I tuck my body-warmed covers around them first, but I can't lay them down easily, or move them over easily.
I figure, I have a big bed, it's easier for me to move over if I have to, yk?
Anyway, about the nursing. Unless your baby is right at breast-level, tucked in with you, you're going to have to wake up to some kind of concious level to get them out of whatever sleeping contraption they happen to be in when they want to nurse at night. Which also means that the baby will have to wake up enough to let you know that they need to nurse. If the baby is right in bed with you, after that initial adjustment period, it is most likely that neither you or the baby will wake up to nurse. The baby will rouse enough to root, and you will rouse enough to help get the nipple to the baby's mouth, but that's all that it takes. And once they're done, they'll just let go, and you don't have to worry about waking them again to get them back in their bed.....which will have cooled down by that point.
I don't know - I may be prejudiced (my folks co-slept with my 4 siblings, the youngest being twins), but I've always felt that the co-sleeper baby beds shouldn't be called such. Because having a baby sleeping in their own little space isn't really co-sleeping (for the breastfeeding & warmth reasons mentioned previosly) - it's just putting their crib within arm's reach. True, your reaction time is much quicker than with the baby in a separate crib across the room or in another room, and the baby still has the benefit of not being alone, and can hear their parents' breathing, but they are still reliant on their own body heat to keep them warm, they aren't getting the carbon dioxide signals from the mother's breathing to keep them regulated, and they have to rouse more fully to nurse.
We use a bedrail on my side of the bed in the early weeks, and I find that that is more than enough security. You find that you become very aware of where your baby is, even when sleeping, and falling isn't much of an issue. If you are concerned about falling, though, you can always get rid of your bed-frame & put the mattress on the floor.